Pyriculol was isolated from the rice blast fungus Magnaporthe oryzae and found to induce lesion formation on rice leaves. These findings suggest that it could be involved in virulence. The gene MoPKS19 was identified to encode a polyketide synthase essential for the production of the polyketide pyriculol in the rice blast fungus M. oryzae. The transcript abundance of MoPKS19 correlates with the biosynthesis rate of pyriculol in a time-dependent manner. Furthermore, gene inactivation of MoPKS19 resulted in a mutant unable to produce pyriculol, pyriculariol and their dihydro derivatives. Inactivation of a putative oxidase-encoding gene MoC19OXR1, which was found to be located in the genome close to MoPKS19, resulted in a mutant exclusively producing dihydropyriculol and dihydropyriculariol. By contrast, overexpression of MoC19OXR1 resulted in a mutant strain only producing pyriculol. The MoPKS19 cluster, furthermore, comprises two transcription factors MoC19TRF1 and MoC19TRF2, which were both found individually to act as negative regulators repressing gene expression of MoPKS19. Additionally, extracts of DMopks19 and DMoC19oxr1 made from axenic cultures failed to induce lesions on rice leaves compared to extracts of the wild-type strain. Consequently, pyriculol and its isomer pyriculariol appear to be the only lesioninducing secondary metabolites produced by M. oryzae wild-type (MoWT) under these culture conditions. Interestingly, the mutants unable to produce pyriculol and pyriculariol were as pathogenic as MoWT, demonstrating that pyriculol is not required for infection.

Caripyrin (trans-5-(3-methyloxiranyl)pyridincarboxylic acid methyl ester, 1), a new pyridyloxirane, was isolated from submerged cultures of the basidiomycete Caripia montagnei. The compound was found to inhibit conidial germination and appressorium formation in the rice blast fungus Magnaporthe oryzae, whereas the infection-related morphogenesis in several other phytopathogenic fungi was not affected. In plant assays on rice, 1 was found to protect plants more efficiently against fungal infection than the structurally related fungal secondary metabolite, fusaric acid. Contrary to the latter, 1 was neither cytotoxic, antibacterial, nor nematicidal. Keywords: antifungal activity; basidiomycetes; Caripia montagnei; caripyrin; Magnaporthe oryzae; plant protection; pyridyloxirane INTRODUCTIONCrop losses world wide due to plant diseases are estimated to account for almost 20% of the major food and cash crops produced. 1 Among the plant diseases caused by fungal pathogens, rice blast, caused by Magnaporthe oryzae, is the most serious disease that infects cultivated rice and therefore a threat to the world's most important food security crop. 2 To enter and colonize the host plant, M. oryzae displays a remarkable morphological and physiological specialization. During the prepenetration phase, the germ tube differentiates into a melanized dome-shaped appressorium after attachment to the leaf surface and germination of the spore. This infection structure is an essential prerequisite for a successful infection, allowing the fungus to penetrate the plant cuticle in a direct manner by mechanical force. 3 Therefore, an intact melanin layer is essential for appressorial turgor generation. Melanin-deficient mutants, for example, ALB1, BUF1 and RSY1, fail to generate appressorial turgor and are nonpathogenic. 4 In M. oryzae, tetrahydroxynaphthalene or trihydroxynaphthalene reductases are targets for the successful and widely used plant protectants, such as tricyclazole and carpropamid. 5 Inhibitors of melanin biosynthesis are excellent examples for plant protectants interfering with the infection-related morphogenesis but not with vegetative growth.To control rice blast, quinone outside inhibiting fungicides are increasingly applied in Japan. Such quinone outside inhibiting fungicides are threatened by resistance development in target pathogens. 6 As M. oryzae is considered as a 'high-risk' pathogen in terms of resistance to quinone outside inhibiting fungicides, inhibition of targets, essential for differentiation processes and pathogenicity, provides interesting alternatives for plant protection strategies.

The new pyridyloxirane caripyrin (I) is isolated from submerged cultures of the basidiomycete Caripia montagnei.